Page:Popular Astronomy - Airy - 1881.djvu/24

10 appear to turn, and which is not far from the Polar Star. The axis being adjusted with great accuracy in that direction, the instrument turns round that axis, and it carries the telescope CD, which, of course, so long as you give it no other motion, retains the same inclination to that axis; but to which you may give another motion, so as to place it in different positions, as C'D' or C"D", directed to stars in different parts of the heavens. The instrument, then, is employed for the purpose of giving evidence as to the motions of the stars. It is used in this manner. The telescope is directed to any one star, and then by turning the instrument round the axis, it is found, that without any alteration in the position of the telescope in relation to the axis, the telescope will follow the star from its rising to its setting. And it is the same wherever the star may be, whether near the Pole, (in which case the telescope is in such a position as C'D', very little inclined to the axis,) or far from the Pole, (in which case the telescope would be much inclined to the axis, as in the position of C"D",) upon turning the instrument round its axis, the telescope still follows the star. This is a fact of accurate observation, for the confirmation of which this kind of astronomical instrument is peculiarly adapted. In this way it is established as a general fact, that all the stars move accurately in circles round one centre.

But there is another important thing to discover—with what rapidity do the stars turn? Do some travel quicker than others? Do some go quickly in one part, and slowly in another? Now, we have most accurate means of determining whether the speed be irregular or uniform, as regards the speed of any one star in any part of its motion—whether the